Prednisolone phosphate (PLP) encapsulated in long-circulating liposomes (LCLs) (LCL-PLP) exerts antitumor activity through the inhibition of tumor angiogenesis. It is known that tumor-associated macrophages (TAMs) play a crucial role in tumor growth as they are actively involved in promoting and maintaining tumor angiogenesis. To gain more insight into the antiangiogenic mechanisms of LCL-PLP, this study aimed to investigate the role of TAM in the antitumor mode of action of LCL-PLP in B16.F10 melanoma-bearing mice. Our results show that TAMs have a pivotal function in the growth of B16.F10 melanoma through the production of pro-angiogenic/pro-inflammatory factors. One of the major inhibitory actions of LCL-PLP on tumor growth is the reduction of the TAM-mediated production of pro-angiogenic factors, whereas production of anti-angiogenic factors by these cells is hardly affected.